Semiconductor fabrication processes are among the most sophisticated and complex processes in manufacturing. Monitoring and evaluation of semiconductor fabrication processes on the circuit structures and other types of structures, (e.g., resist structures), is necessary to ensure the manufacturing accuracy and to ultimately achieve the desired performance of the finished device. With the development trend in miniature electronic devices, the ability to examine microscopic structures and to detect microscopic defects becomes crucial to the fabrication processes.
Various technology and methods of defect inspection on patterns or structures formed on semiconductor wafers have been developed and employed with varying degrees of success. For example, an optical inspection method employs an optical inspection device such as an optical microscope for inspecting pattern shapes for defects. As pattern shapes and structures become finer, it is difficult to detect defects using an optical method due to its wavelength limitation. Accordingly, there has been proposed a method of defect detection on fine pattern shapes using an electron beam.
Electron beam inspection uses an SEM (scanning electron microscope) to detect electrical and/or physical defects on a semiconductor wafer. In general, SEM is a type of electron microscope that produces images of a sample by scanning it with a focused electron beam. One application of electron beam inspection is to perform inspection of conductive features such as interconnects and through-vias to detect electrical defects such as electrical shorts, electrical opens or resistive shorts/open, etc. Specifically, the interaction of the electron beam with electrons in the sample generates a number of signals in varying intensities, which may contain information about the sample's features (e.g., its surface topography and composition). The type of signals produced by a SEM includes secondary electrons, back-scattered electrons, x-rays, specimen current and transmitted electrons, etc. Defects may be detected based on these signals. For example, one of the commonly known inspection techniques, sometimes called the voltage contrast technique, employs secondary electron signals for defect detection. Voltage contrast inspection operates on the principle that the potential differences in the various locations of a wafer under examination cause differences in secondary electron emission intensities. Thus, the potential state of the scanned area may be acquired as a voltage contrast image such that a low potential portion of, for example, a wiling pattern might be displayed as bright (intensity of the secondary electron emission is high) and a high potential portion might be displayed as dark (lower intensity secondary electron emission). Alternatively, the system may be configured such that a low potential portion might be displayed as dark and a high potential portion might be displayed as bright. A secondary electron detector is employed to measure the intensity of the secondary electron emission that originates at the path swept by the scanning electron beam. A defective portion can be identified by, for example, comparing the defective voltage contrast image and the defect free image.
Defect detection using an electron beam has a problem in that the readout bandwidth (and therefore throughput) and the signal to noise are severely hampered by the large substrate parasitics, e.g., a large parasitic capacitance. As a result, the time necessary for large area inspection is signification and may take several days and even months. Accordingly, it is desirable to improve electron beam inspection techniques and particularly to increase the throughput of electron beam inspection of semiconductor wafers and other substrates.